1. Field of the Invention
This invention pertains in general to solid oxides for fuel cells and electrolyzers. Specifically, the invention relates to the use of single-crystal zirconia for the electrolytic production of oxygen and carbon monoxide from carbon dioxide, particularly in the atmosphere of Mars.
2. Description of the Prior Art
About 95 percent of the mass of the atmosphere of Mars is carbon dioxide. Accordingly, it has been a goal of space technology to develop a conversion process for separating oxygen from the carbon dioxide and to provide the oxygen in forms useful for propulsion and/or life support.
A prior-art process for accomplishing that goal has been to combine hydrogen with the carbon dioxide to produce methane and water, and then electrolyze the water to recover some hydrogen and produce the oxygen. The first stage of the process requires a catalytic reactor, while the second stage is carried out through traditional water electrolysis. The process requires a continuous feed of hydrogen supplied from earth and stored for use, which is very hard to achieve under all conditions. The inherent difficulty with hydrogen storage lies at the level of atomic physics, rather than engineering, in the fact that hydrogen is the smallest atom and forms the smallest molecule in nature. Therefore, hydrogen easily penetrates microscopic gaps in seals and even penetrates atomic lattices of metals. Thus, all known methods for storing hydrogen are heavy, and all are temporary, that is, they leak at appreciable rates.
The difficulty of storing hydrogen has been of concern to both NASA and the fuel cell community and has been a subject of research for decades. The issues of storage-system mass, volume and leak rates have been of primary concern even for terrestrial applications. No storage system offers a mass efficiency better than 20 percent; that is, 80 percent of the system weight consists of tankage or something other than hydrogen. At the same time, according to a recent NASA study, the minimum boil-off rate achievable with current technology is about 1.5 percent of hydrogen per day. On the basis of that estimate, the prospects for transporting hydrogen to Mars for oxygen and fuel production are negligible because the journey from Earth to Mars takes about 180 days.
Another known process for oxygen production accomplishes carbon dioxide conversion directly through solid-oxide electrolysis. The reaction takes place at high temperatures in a single stage according to the equation EQU 2CO.sub.2 =2CO+O.sub.2.
No hydrogen or other consumables from Earth are needed to support this carbon dioxide conversion process; in addition, the carbon monoxide byproduct can also be used as a fuel.
Solid-oxide electrolysis is based on a discovery by Walther Nernst that certain ceramic oxides can conduct oxygen ions much as metals conduct electrons. This phenomenon, called "second conductivity" by Nernst, requires a high operating temperature for ionic conductivity to take place and is the principle underlying the use of solid oxides for fuel cell and electrolytic applications. Ionic conductivity is different from conduction of electrons in metals. Electrons in metals move relatively freely in electron clouds, whereas oxygen ions in a solid oxide hop between holes in the crystal lattice that are forced open by a dopant. A dopant is generally needed anyway to stabilize the crystal lattice against phase changes that would otherwise occur during heat up.
A typical solid oxide used in fuel cells and electrolyzers is cubic zirconia with yttria dopant (Y.sub.2 O.sub.3), generally in a 8-10 percent yttria mole fraction. Yttria doped zirconia is known by the trade name "zircon" when used as artificial diamonds in costume jewelry. Fuel cells and electrolyzers employ the material in ceramic form, a polycrystalline agglomerate of randomly oriented crystals.
The properties of stabilized zirconia ceramics have been utilized in various fields of technology, from oxygen sensors for fuel-air ratio optimization of automobiles and furnaces; to solid-oxide fuel cells for noiseless and clean power generation from chemical energy; and oxygen pumps for solid state oxygen separation. The oxygen-ion conduction properties of stabilized zirconia used in typical oxygen sensor, fuel cell, or oxygen pump applications are well understood based on electrochemical-cell theory.
The polycrystalline solid-oxide electrolyte used in prior-art applications is formed into thin-sheet ceramics, either flat or of tubular geometry, with porous electrodes on both sides of the electrolyte. The role of the cathode in the oxygen-generation application of interest is to facilitate the division of CO.sub.2 molecules into CO and O and to provide two electrons to each oxygen ion. The electrolyte allows the voltage difference across the electrodes to pump the oxygen ions toward the anode, and the anode accepts the electrons from the oxygen ions and permits them to combine into O.sub.2 gas. The operation is efficient if surface and bulk resistivities are low, so that much of the voltage drop is invested in driving the chemistry. Typical system efficiencies of fuel cells are above 50 percent, and the same is achieved for solid oxide electrolysis.
Each oxygen ion within the electrolyte is doubly charged, so ionic current and oxygen production rate are directly related. An oxygen production rate of 0.1 kg/hr, for example, corresponds to an ionic current of 335 Amps. The potential needed to drive CO.sub.2 electrolysis is about 0.8 Volts, and surface potentials add about 0.3 Volts per electrode. Thus, the minimum power needed to produce 0.1 kg of oxygen per hour is about 335.times.1.4=469 Watts. In practice, bulk resistivity of the electrolyte adds a bit to the power requirement, but bulk resistivity is low if the electrolytic sheets are thin and have a relatively large area.
Many solid-oxide cells have been developed for fuel-cell applications, such as zero-emission power sources for generators and vehicles. Solid-oxide electrolysis cells are essentially solid-oxide fuel cells with reversed polarity. In fact, solid-oxide electrolysis cells can be run as power generators by flowing air or oxygen on one side of the electrolyte and carbon monoxide on the other. Therefore, system-mass considerations and the knowledge already generated by industrial developments in the field provide good arguments in favor of solid-oxide electrolysis, rather than other known processes, as a means for producing oxygen on Mars.
While the polycrystalline ceramics used in prior-art solid-oxide applications provide the electrochemical vehicle for the electrolytic process for oxygen production, in practice they are not acceptable for extra-terrestrial applications, where physical strength and reliability during thermal stresses are essential. The solid-oxide electrolyte utilized for production of oxygen in Martian atmosphere must be capable of withstanding the shocks of transportation and the stresses caused by temperature cycles from the 900-1,000.degree. C. operating temperatures during electrolysis to below zero .degree. C., the typical temperature to which equipment is subjected during Martian nights. Polycrystalline ceramics shrink and tend to crack under severe heat exposure. They cannot be machined to strict tolerances because of their porous structure and are too brittle for pressing to form a seal. Therefore, glues must be used to assemble the hermetic cells required for the electrolytic production of oxygen from CO.sub.2. The resulting structures are relatively fragile and cannot be disassembled without totally disabling them. These problems, which are serious for commercial applications on Earth, appear insurmountable for extraterrestrial applications.
Therefore, there is still a need for a viable solid-oxide material for fuel-cell and electrolytic applications under harsh operating conditions. The present invention discloses a novel approach that satisfies these requirements for any application, and that is particularly suited for the cyclical production of oxygen from the carbon dioxide atmosphere of Mars.